1. Field of the Invention
The present invention relates, in its more general aspect, to a synchronous electric motor particularly, but not exclusively, used in smoke suction fans mounted in wall boilers.
In particular, the invention relates to a driving method of a synchronous electric motor comprising a rotor equipped with a permanent magnet, which is rotation-operated by the electromagnetic field generated by a stator being equipped with pole pieces and the relevant windings.
2. Description of the Related Art
As it is well known to the skilled in the art of this specific field, wall boilers are equipped with smoke extraction fans, commonly called exhaust fans, generally operated by asynchronous electric motors.
Fan models operated by synchronous electric motors have been only recently introduced.
Wall boilers generally comprise, besides a burner, a driving electronic box allowing the burner to be turned on and off, eventually by means of a timer, as well as the increase or decrease of the plant circulation water temperature to be regulated.
Smokes produced by the burner have different physical features according to the boiler operation speed and thus, in order to optimize the boiler operation, the fan should adjust the smoke suction to this operation speed.
In particular, when turning on, an initial thermal-transient step occurs, after which a steady-state step begins, which can be defined as a fixed-displacement state. During the initial step, the smokes being produced have a higher density and the fan should thus change the motor operating conditions by sucking more in order to extract smokes and allow the boiler to operate in the best way.
Obviously, the same thermal transient occurs if the plant temperature is changed.
It is thus typical to position in the plant, downstream the fan, a smoke flow rate regulation lock which, according to the boiler position and more particularly to the flue lodgement and flow resistance level, allows a constant-in-time smoke mass flow rate to be kept.
This lock undergoes, for a good operation, a constant and accurate maintenance without which a burner shutdown and/or an extremely dangerous toxic smoke saturation in the boiler room and eventually in adjacent rooms is threatened.
Too sharp changes of the load flow rate can also cause a temporary motor shutdown, which would require a sudden manual or automatic intervention for a new starting.
It is also convenient that the electric motor always operates at a speed approaching the highest efficiency speed, defined by an operation point corresponding to the lowest power absorbed by the line, also resulting in energy saving for the user.
In order to obtain a speed approaching the highest efficiency speed, it would be very useful to have a measure of the load, i.e., of the smoke flow rate, available any moment.
Different devices for measuring the flow rate, even through continuous surveys, are known in the prior art.
Generally, these flow meters are devices being positioned in correspondence with the fan deliveries and, by exploiting different principles, determining the gas volume flowing through a given section in a predetermined time, in particular a differential manostat can be for example provided, positioned between the fan suction and delivery.
All these meters, although satisfying the aim of providing a flow rate measure, thus have some drawbacks.
First of all, a certain area, wherein they must be housed, is to be provided on the delivery.
Moreover, it must be underlined that, in order to check the good operation thereof, these devices must undergo periodical precautionary maintenance activities.
Other solutions can provide the use of current sensors in order to indirectly determine the fan flow rate by sensing a higher or lower current absorption by the stator windings.
Nevertheless, this solution does not provide precise and reliable measures.
A further prior art solution is described in the European patent application no. 0 403 806 relating to a centrifugal pump or fan for letting a controlled-temperature fluid flow, particularly in heating systems. Sensors for determining the fluid flow rate and temperature sensors for determining the fluid temperature are provided. A control device being associated to the electric motor processes the values received by the sensors for the motor driving in order to obtain an almost constant fluid temperature.
Also this solution requires expensive sensors to be managed, complicating the structure of the fan and of the device for driving the relevant electric motor.